1. Field of the Invention
This invention relates to the synthesis and use of double stranded polymers to separate chemical mixtures.
2. Description of the Related Art
The ability to separate chemical mixtures into their individual components is a billion dollar industry. Speed, resolution, efficiency, reproducibility, repeatability and low cost are all factors that determine a good separation method. Current methods to separate chemical mixtures include chromatography and electrophoresis. These techniques are essential for clinical analysis, for biotechnology, environmental analysis, and for drug development. Chemical separation primarily entails the chemical/physical interaction of a stationary surface (a stationary phase) with chemicals (in a mobile phase) that flow by the surface of the stationary phase.
Chromatography achieves the separation of chemical mixtures by the use of a mobile phase and a stationary phase. The mixture is injected into the mobile phase and the mobile phase then flows over the stationary phase. The different interactions of the individual components with this combination of phases creates a separation. If a neutral component has a polar character then it will be retained longest by a polar stationary phase. Neutral non-polar solutes will be retained best by a non-polar stationary phase. The mobile phase can be liquid, a gas or a supercritical fluid. The stationary phase can be a packed or wall coated standard or capillary column. The type of mobile phase determines the name of the method, e.g. liquid chromatography, gas chromatography . . . etc.
A conductive polymer, namely single strand polyaniline, has been coated on a glassy carbon stationary phase to achieve charge controlled chromatography. The coating allowed the number of ion exchange sites in the stationary phase to be controlled instead of being fixed as in ion chromatography and inorganic anions and organic acids were separated. However, a drawback of coating the single strand polyaniline on the stationary phase is that the single strand polyaniline does not have a wide enough pH range to be usable.
Electrophoresis creates separations of charged molecules. Electrophoresis is principally used for the separation of biological molecules and remains a standard biological tool. Charged molecules can be separated in slab gels by the application of an electric field. Capillary electrophoresis separates charged molecules according to their electrophoretic mobilities in an electric field. Generally, the separation compartment is a narrow fused silica capillary filled with an electrolyte solution. The electric field is applied with an external voltage source between two electrodes in small vials in contact with the electrolyte solution at both ends of the capillary. The sample is introduced either hydrostatically or electro migration as a narrow zone at one end of the capillary. Typically, UV detection takes place at the other end of the capillary.
Fused silica is the typical capillary material used in capillary electrophoresis because it is inexpensive, easy to fabricate into capillaries with internal diameters in the 10-300xcexcm ranges, possesses optical transparency for both UV and visible spectrums, is mechanically strong and is flexible when coated with polyimide. However, the material properties of fused silica presents some drawbacks when used in capillary elecrophoresis. For example, the surface silanol groups of the fused silica behave as a weak acid, ionizing in water, with a broad titration curve in the pH 3.9 to 9 region. These surface anionic groups induce both electro osmotic flow (EOF) and solute wall interactions to occur. Solute wall interactions typically occur with cationic proteins that electrostatically bind to the silica. Reversible interactions between such analytes and the capillary surface worsen the separation profile, broadening the peaks and decreasing reproducibility, while irreversible interactions can destroy the flow profile entirely. Attempts to reduce these interactions include the use of extreme pH buffers (very high and very low), the use of additives and modification of the capillary surface.
Several coatings have been applied to the surfaces of a capillary to address the drawbacks associated with the use of fused silica as the capillary material in capillary electrophoresis. For example, nonconducting polymers have been adsorbed to the surface of a fused silica capillary. However, the prior art polymeric coatings that have been adsorbed to the surface of a fused silica capillary are typically unstable.
The present invention provides a coating for the stationary phase of a chemical separation system, the coating comprising a double stranded conductive polymer that is efficient, effective and overcomes the drawbacks associated with existing chemical separation systems.
Broadly, the invention comprises a double stranded conductive polymer functioning as the stationary phase of a chemical and/or biological separation system. The double stranded conductive polymer provides controllable interactions between the polymer system of the stationary phase and the chemicals and/or biological analyte in a carrier stream. The invention further comprises a chemical separation system comprised of stationary phase comprising a double stranded polymer.
The double stranded conductive polymer used in the invention comprises a linear strand of polyaniline and a linear strand of a polyelectrolyte twisted together to form a macro-molecule. The polyaniline strand can be modified to predictably change its hydrophobicity and/or its color when the pH and/or the electrochemical potential within the separation system is changed. The modification of the polyaniline strand controls the analyte-surface interactions to improve the separation. The linear strand of polyelectrolyte provides the properties suitable for non-aggressive interactions with the analyte or carrier stream.
The double stranded conductive polymers can be used as a part of the stationary phase in a chromatographic column, as a coating on the inner surface of a capillary for separation by capillary electrophoresis, as part of a filtration membrane, as a component in gel electrophoresis and/or coated on or admixed with particulate material packed in a column or the like. The double stranded conductive polymer has the chemical structure that is suitable for selective interaction with molecules dissolved in a carrier fluid that flows by the polymer to effect chemical separation of the components in the mixture.
The double stranded conductive polymers used for chemical separation belong to a class of polymers comprising a molecular complex of two strands of polymers: (1) a xcfx80-conjugated polymer such as polyaniline, plypyrrole, polythiophen, poly(phenylene vinylene), etc. and (2) a polyelectrolyte such as poly(acrylic acid), poly(methylvinylether-co-maleic acid), poly(butadiene-co-maleic acid), poly(vinylsulfonic acid), poly(styrenesulfonic acid), poly(methacrylic acid), poly(L-glutamic acid), poly(L- Asparic acid), etc. The two strands of the molecular complex are bonded non-covalently for most of the applications, although crosslinking between the two strands is also possible. The synthetic process (the template-guided synthesis) allows the control of solubility, conformation, and the morphology of the polymer and thus provides advantageous properties for chemical separation applications. The two strands of polymers in the molecular complex are likely to be non-covalently bonded in a side-by-side arrangement thus they are referred to as double-stranded polymers, although the actual structure of the complex could be somewhat random.
A double stranded conductive polymer was coated on the inner surface of a glass capillary in a capillary electrophoretic separation apparatus. The electro osmotic flow (EOF) carried the chemical mixture through the capillary. Due to the influence of the xcfx80-conjugated polymers coated on the capillary wall, the different types of molecules in the mobile phase are separated by their difference in elution time. The present invention embodies coatings for improving the analysis of organic and inorganic species by chromatographic and electrophoretic techniques. These experiments demonstrate the beneficial molecular interaction between the stationary and the mobile phases. The same molecular interactions can be used for liquid chromatography, HPLC, of thin-layer chromatography for chemical analysis. The double stranded conductive polymers are also useful for large-scale separation of chemicals or drugs when it is used as a component in preparative-scale chromatography or as part of a membrane for selective filtration of chemicals.
The double stranded polymer used for the preferred embodiment comprises two components: (1) a polyaniline molecule, and (2) a polyanion. These two strands of polymers are bonded by non-covalent intermolecular interactions to form a stable molecular complex. Examples of the polyanion in the polymeric complexes are poly(stryrenesulfonic acid), poly(acrylic acid), poly(methacrylic acid), poly(2-acryamido-2-methyl-1-propenesulfonic acid), and poly(methylacrylate-co-acrylic acid), poly(butadiene-co-maleic acid), poly(glutamic acid), poly(aspartic acid), etc.
Another advantage of using the double stranded conductive polymers in a chemical separation system is the relative ease in functionalizing the polymer to adjust material properties to meet the demand for practical applications. The double stranded conductive polymers are synthesized to be soluble in water, or soluble in organic solvents, or suspended in latex to satisfy the demands of coatings applications. Certain functional groups of the double stranded conductive polymer provide strong adhesion to metals and other polymers, an advantageous property for coatings application for the stationary phase in the separation system.
The double-strand conductive polymers are synthesized by a method that encourages the formation of molecular complexes. In the first step, aniline monomers are absorbed onto a polyanion chain dissolved in solution. The resulting adduct, polyanion:(aniline)x has signatures that can be monitored and verified. In the second step, the attached aniline monomers are oxidatively polymerized to form the polymeric complex.
The adduct of polyanion:(aniline)x may take the shape of a tight coil or extended chains. The shape of the adduct controls the morphology of the polymerized product. A tight-coiled adduct results in globular polyaniline complex, while an extended chain adduct results in thin fibers of the double-strand complex aggregates (100 nm diameterxc3x975 micron length). Thus the polyanion functions as a template during the chemical synthesis, and the template becomes the second strand of the xe2x80x9cdouble-strandxe2x80x9d polyaniline after polymerization.
That is the template guided synthesis allows for controlling the morphology (e.g., fibrous or globular) of the complex as well as the conformation (e.g., coiled or extended chain, helical or sheet conformation) of the polymer. Because the polymer has delocalized electrons on the polymer backbone, the van der Waals and electrostatic interaction of the polymer with the mobile phase can be quite different from the conventional materials for stationary phase. This feature is advantageous for separation of proteins, DNA, and drugs because the delocalized binding between the polymer and the analytes can be designed to be specific enough to be selective among the molecules that are otherwise difficult to be separated.